Patent GB-1018586-A (GB)
Title: Improvements in or relating to the detection of motor vehicles and like metal masses
IPC: B61L1/10
Filed: 1962-08-02
Abstract: 1,018,586. Locating conductors. GENERAL PRECISION Inc. Aug. 2, 1962 [Aug. 4, 1961], No. 29734/62. Heading G1N. [Also in Division H3] In apparatus, Fig. 1, for sensing the presence of a metallic mass e.g. a motor- or railroadvehicle, the output of a loop oscillator 11 comprising in its tank circuit an inductive loop 12, and the output of a reference oscillator 13, are passed to a beat frequency detector circuit 14, so that when the frequencies of the oscillators diverge due to the presence of a metallic mass in the magnetic field associated with loop 12, a signal of frequency equal to the difference of the oscillator frequencies is fed via a frequency selective amplifer 15 to a detector circuit 16 and thence to a switch circuit 17 to control a load device 18, means being provided to enable the switch circuit to be maintained in its operated condition by a smaller voltage than that required to operate it. Preferably the switch circuit 17 is connected to the loop oscillator 11 through a second detector circuit 19, so that the switch circuit operates if the loop oscillator fails. In Fig. 2, (not shown) both loop oscillator 11 and reference oscillator 13 comprise resistance coupled transistors 37, 40 and 21, 22 the latter of each pair being operated as an emitter follower. The tank circuit of the loop oscillator comprises the detector loop 12 in parallel with series connected capacitors 43, 44; positive feedback to the emitter of transistor 37 is provided by direct coupling to the series connection point of capacitors 43, 44 while negative feedback to the base of 37 is supplied via resistor 46. Positive feedback in the reference oscillator is provided via a capacitor 32 from a tapping on inductance coil 31 in the conventional tank circuit 29, and negative feedback, which may be omitted, is supplied via resistor 33. The outputs of the oscillators 11, 13, which are of equal frequency in the absence of a metallic mass in the field of loop 12, are fed via resistors 49, 50 to the base and collector respectively of a transistor 48 which constitutes the beat frequency detector 14, and which produces an A.F. output when its input frequencies differ. This A.F. output is supplied via a low pass filter network 52, 53 a variable capacitor 54 and a resistor 55 to the frequency selective amplifier 15 comprising cascaded transistors 56, 62, the output from the latter, which is connected as an emitter follower, being rectified in a voltage doubling circuit 16 to produce a negative potential at point 71; over the operating portion of its characteristic, amplifier 15 produces an output proportional to the input frequency. The output from loop oscillator 11 is also passed to a detector 19 comprising a transistor 73 which normally produces a square wave of constant amplitude which is supplied to voltage doubling network 78, 79, 80, 81 to produce a positive potential at point 82. This positive potential forms the normal bias for a pair of transistors 84, 85 connected as a Schmidt Trigger. Transistor 85, which is normally conducting, is directly connected to an emitter follower 86 whose output is fed to a transistor 87 operating as a switching amplifier and directly coupled to a normally conducting transistor 88 having the load device, such as relay winding 100, in its collector circuit. The appearance of the negative potential at point 71, or the disappearance of the positive potential from point 82, renders transistor 84 conductive and transistors 85, 88 non-conductive. The switching circuit is rendered partially self-sustaining by hysteresis feedback via rheostat 126; alternatively, the oscillators 11, 13 may be passively coupled by a resistor 127. Figs. 4 and 5 (not shown) depict alternative loop oscillator circuits comprising inductive loops 12 with earthed centre-taps; Fig. 5 discloses a push-pull arrangement. In Fig. 6 (not shown), the loop 12 is asymmetrically disposed relative to a gate 131 so that a vehicle approaching the gate in the direction 130 will cause detector 133 to operate an actuation device 132 to open the gate, while a vehicle approaching from the opposite direction will not do so. In Fig. 7, (not shown), the loop 12 is positioned between the rails 137 of a railroad track. When the first axle 138 of a truck passes over the loop, the detector 133 operates to actuate signals, crossing gates etc.; the effect on loop 12 is somewhat smaller when the truck is in the position shown, but the self-sustaining property of the switching circuit maintains the switch operated.